The present invention relates to method of manufacturing a color filter by means of a color resist and an alignment mark to be used for this color filter manufacturing method.
Nowadays, applications of the solid-state imaging devices including CCD (Charge Coupled Device) are spreading to digital still cameras and image input terminals for personal computers and so on in addition to video movies. Under these circumstances, there is a growing demand for the use of color filters of a primary color system in place of color filters of a complementary color system which have conventionally been in the mainstream. The color filters of the primary color system have been in the spotlight for their very simple signal processing performance and extensive applicability. However, the manufacturing method thereof has become difficult to be adopted due to an extremely large volume shift (a maximum of 1 .mu.m in lateral direction) through a dying process in such a big microstructural progress of the solid-state imaging devices.
Color filters manufactured by the dying method which have conventionally been in the mainstream are required to achieve a rapid improvement for the reason that the resolution and uniformity are coming close to their limits and for the reason that the dying of the primary color system has a slight lack of process stability as compared with the dying of the complementary color system. Under such circumstances, fabrication of a color filter by means of the aforementioned color resist, which has reached to a certain level in terms of the dispersibility and processability of pigment, has come to attract latest attentions although some problems are currently left in terms of the resolution and so on. Expecting the future improvement in performance, the fabrication of the color filter by means of the color resist will produce a great effect not only on improvement in characteristics of the solid-state imaging devices but also on enhancement of manufacturing yield and a reduction in manufacturing cost. It is to be noted that the color resist is a positive or negative resist obtained by dispersing a pigment in a transparent base material, or a positive or negative resist obtained by uniformly dissolving a dye in a transparent base material.
According to the color filter manufacturing method using the color resist as described above, there is a problem in terms of alignment of a photomask used in exposing a color resist in a blue filter forming process as described in detail below. That is, in an exposure apparatus such as a stepper, an alignment mark on a substrate is normally detected by a red light of a He--Ne laser or the like in an alignment process. The red light is used for avoiding the activation of the resist. On the other hand, there is the fact that the blue color resist has a very low transmittance of the red light.
Normally, as shown in FIGS. 6A and 6B, the alignment of a photomask (not shown) for exposure in the blue filter forming process is executed by detecting an alignment mark (groundwork alignment mark) 2 formed on a substrate 1 by means of the red light through an overcoat film 3 that functions as a flattening film and through a blue resist layer 4. However, due to absorption of the red light by the blue resist layer 4 in addition to low contrast of the groundwork alignment mark 2, a signal light that is reflected on and diffracted by the groundwork alignment mark 2 and enters a detector (not shown) becomes very faint. Therefore, the alignment of the photomask in the blue filter forming process is very difficult to be executed. It is to be noted that the reference numeral 5 denotes a filter of another color (red, for example) and the reference numeral 6 denotes a blue filter.
In order to solve the aforementioned problem, there have been proposed various methods such as a method for improving the groundwork alignment mark detection performance by improving an alignment optical system as disclosed in the prior art references of Japanese Patent Laid-Open Publication No. HEI 6-260390 and Japanese Patent Laid-Open Publication No. HEI 4-133349, a method for removing the color resist on the groundwork alignment mark in a coating stage, as disclosed in the prior art references of Japanese Patent Laid-Open Publication No. HEI 3-163403 and Japanese Patent Laid-Open Publication No. HEI 8-297206 and so on. However, it is not possible for any of the above methods, each of which requires improvement of existing apparatus and introduction of new apparatus, to assure a high throughput at low cost. Therefore, a more simple method for solving the problems of the alignment in the blue filter forming process is demanded.
Lately, a color filter manufacturing method for improving illuminance of the alignment mark without requiring the improvement of the existing apparatus nor the introduction of the new apparatus as described above has been proposed (Japanese Patent Laid-Open Publication No. HEI 9-96712). According to this color filter manufacturing method, as shown in FIG. 7A, a flattening layer 13 is formed to cover the alignment mark 12 on a substrate 11 and a transparent layer 14 is formed on the flattening layer 13 in a position just above the alignment mark 12. Next, a color resist layer 15 is formed on the flattening layer 13 and the transparent layer 14 so that the thickness of the color resist layer 15 on the transparent layer 14 is thinner in a portion on the transparent layer 14 than in the other portions. Then, as shown in FIGS. 7A and 7B, a color filter 17 comprised of a pattern of the color resist layer 15 is obtained by detecting the alignment mark 12 by applying an alignment light 16, performing positional alignment of a photomask (not shown) with the substrate 11 and thereafter subjecting the color resist layer 15 to exposure to light and development. With this arrangement, by virtue of the thin film thickness of the color resist layer 15 just above the alignment mark 12, illuminance of the alignment mark 12 can be improved even when the color resist layer 15 is a blue resist layer and the alignment light 16 is a red light.
The color filter manufacturing method shown in FIGS. 7A and 7B basically follows the conventional method, in which for forming an alignment mark is just the same time of forming a layer (polysilicon electrode or a light-shielding layer, for example) for trimming the light-sensing aperture, in the field of solid-state imaging devices and the like, and the method is regarded as a measure for increasing the illuminance of the alignment mark without process improving cost and significant process change.
However, the above prior art color filter manufacturing method has the problems as follows. That is, as shown in FIG. 7A, the thick flattening layer 13 is formed between the color resist layer 15 that serves as a layer to be aligned and the alignment mark 12. When such a thick flattening layer 13 is formed between the layer to be aligned (the color resist layer 15) and the groundwork alignment mark (the alignment mark 12), a great distance is made between the top surface of the layer to be aligned and the top surface of the groundwork alignment mark. Therefore, particularly when a laser light is used as the alignment probing light, there is a problem that an alignment error tends to occur due to refraction or ghost.
Furthermore, there is a still remaining problem in regard to improvement in contrast of the alignment mark necessary for mark recognition of the exposure apparatus. That is, assuming that the alignment light 16 has a luminosity I, that the color resist 15 has an optical-absorption coefficient a, that the color resist 15 has a film thickness d.sub.1 /2 (d.sub.1 : film thickness of the flat portion of the color resist 15) just above the alignment mark 12, that the substrate 11 has a reflectance R.sub.0 and that the alignment mark 12 has a reflectance R.sub.M, then the contrast C of the alignment mark 12 is expressed by C.apprxeq.Ie.sup.-.alpha.d1 .vertline.(R.sub.0 -R.sub.M).vertline.. Therefore, the contrast C of the alignment mark 12 depends on the reflectance R.sub.M of the alignment mark 12. However, as described above, the groundwork alignment mark is normally formed with a silicon compound layer that determines the light-receiving aperture of the polysilicon electrode, light-shielding layer or the like. Therefore, the reflectance R.sub.M is not so significantly different from the reflectance R.sub.0 of the substrate 11 formed of silicon or silicon oxide, so that the contrast C of the alignment mark 12 becomes not so strong.
Furthermore, if the transparent layer 14 has a shape identical to the shape of the alignment mark 12 and they are vertically stacked, then a light that penetrates either the transparent layer 14 or the alignment mark 12 is mixed with a light that penetrates peripheral portions of both of them, possibly causing generation of a false signal of a mark signal of the exposure apparatus.
That is, according to the above prior art color filter manufacturing method, there are the remaining problems of the alignment error due to the great distance between the color resist layer 15 and the alignment mark 12, the reduction in contrast of the alignment mark 12, the generation of the false signal of the mark signal of the exposure apparatus and so on, and thereby a high alignment accuracy cannot be assured when the illuminance is reduced due to the absorption of the red alignment light by the blue resist.